1. Field of the Invention
The invention relates to the alignment of vehicles, and in particular to the alignment of heavy-duty vehicles such as semi-trailers or tractor-trailers. More particularly, the invention relates to an apparatus and method to quickly, accurately and repeatably measure the alignment of a heavy-duty vehicle.
2. Background Art
Proper alignment of vehicles is important for a number of reasons, but its primary purpose is to increase tire life. This is because the tires of improperly aligned vehicles typically wear faster. Proper alignment also is important for semi-trailers and tractor-trailers, that is, those heavy-duty vehicles having a tractor which pulls a trailer. The trailer is attached at its front end to the rear of the tractor via a kingpin which is mounted on and depends from the bottom of the trailer. Most trailers have at least a pair of axles for mounting wheels/tires, with the rearwardmost tandem axles being mounted on a subframe assembly commonly referred to as a slider. The slider is movable fore and aft beneath the trailer along rails which are mounted on and depend from the trailer frame or cross members, to enable repositioning of the axles for adjusting the load on the axles to meet various road and bridge laws.
Three measurements are important in the alignment process for semi-trailers. The first is the alignment of the trailer frame rails relative to the trailer kingpin. This measurement is typically made in the factory, when the frame rails are to be welded to the trailer. The second is alignment of the front slider axle relative to the kingpin, commonly referred to as the “thrust angle,” which is measured both in the factory and in the field. The third is the alignment of any other slider axles relative to the front axle, which is also measured in both the factory and in the field.
Manufacturers and dealers of semi-trailers have for many years recognized the difficulty of achieving an accurate and repeatable alignment system. For example, to measure the alignment of the trailer frame rails, the frame rails are typically placed up against a stop in a fixture. A standard tape measure then is used to measure the placement of the rails before they are welded to the trailer frame. This system fails to provide alignment measurements that are accurate or repeatable.
As to measurement of the thrust angle and measurement of the alignment of the front and rear axles relative to one another, one popular method of alignment involves the use of lasers, which are attached to the trailer rear tires or wheel rims. The laser is directed to a crossbar typically temporarily mounted on the kingpin of the trailer. However, the laser system is not repeatable due to the fact that tire and rim surfaces vary enough to cause the alignment process to fall outside of the tolerances required to improve tire life.
Another alignment method commonly used has been to attach a simple tape measure to a conventional fish scale. The tape measure is extended between the kingpin and an outboardly extending reference structure threadably mounted on the threaded spindle end of the axle. Although the use of the fish scale is intended to regulate the tension in the tape measure to increase the accuracy of the process, too much tolerance occurs in this measuring method as well. This is because it is difficult to achieve the proper tension in the tape using the fish scale and at the same time read the measurement on the tape, thus adversely impacting the repeatability of the method. Moreover, the reference structures of the prior art fail to achieve repeatable proper positioning and alignment with the axle center line.
As a result, a longstanding need has existed in the art for an alignment measuring apparatus and method that overcomes the disadvantages of prior art apparatus and methods, and which is economical, easy to use, accurate and repeatable.